Wireless energy transfer often requires that the source of energy be located close to the device to which the energy is being transferred. Energy can be transferred using near-field wireless transmission techniques such as induction based on magnetic fields in close proximity to the antenna. But the amount of energy transfer drops off significantly with distance. For example, beyond one to three feet, the energy transfer is negligible. To address this issue, near-field technologies have employed large parabolic antenna systems, such as satellite dishes, to focus the electromagnetic fields. But this technique can be difficult to implement due to costs and logistics of having such large equipment that is difficult to aim. And the target device to be charged cannot be moved while still being charged.
Transferring energy wirelessly in a far field—a large distance from the antenna—can be difficult because of the drop in energy transfer due to distance and the difficulty in aiming a beam at a target device that is far from the antenna. Global positioning systems (GPSs) can allow a device determine its location and velocity by measuring the time it takes to receive radio signals from four or more overhead satellites. Due to various error sources, standard GPS yields position measurements accurate to approximately ten meters. Differential GPS (DGPS), which enhances the system through a network of fixed, ground-based reference stations, has improved accuracy to about one meter. But meter-level accuracy is not sufficient to support accurate far-field wireless energy transfer, particularly for moving target devices.